Work machines such as, for example, wheel loaders, motor graders, track-type tractors, dump trucks, and other types of machinery are used to perform a variety of tasks associated with an industry such as, mining, construction, manufacturing, transportation, or any other such industry. Generally, work machines include a power source such as, for example, a diesel engine, a gasoline engine, a natural gas engine, or any other type of power source that provides power required to complete these tasks. To efficiently perform these tasks, the work machine may utilize a transmission that is capable of transmitting energy produced by the power source, in the form of torque, over a wide range of speeds. In some work machines, the transmission may include a generator coupled to the power source to provide an electrical power output for one or more electric motors. The motors may be connected to ground engaging traction devices to propel the work machine.
The electric motors coupled to the traction devices may include, for example, switched reluctance (or variable reluctance) motors. While these types of motors may provide the variable speed capability required to effectively propel a mobile machine, torque output over the broad range of speeds may be limited. For example, for a series wound switched reluctance motor, torque output at low speeds is typically very high and nearly constant. However, as rotor speed increases, the high current required to maintain the electromagnetic force necessary to maintain constant torque output cannot be provided, due to the electrical time constraints of the charging and discharging of the phase coils. Thus, it may be advantageous to provide a system to allow additional current flow through the phase coils at higher speed to provide a constant torque output over a wider range of speeds.
One method to maintain the torque output of a switched reluctance motor is described in Published U.S. patent application No. 2004/0217668 (hereinafter referred to as the '668 publication), which discloses a switched reluctance electrical machine including a salient pole stator, a salient pole carrier configured to move relative to the stator, and a plurality of coils. Each coil includes a “tap” to alter the effective number of turns in the coil, based on the speed of the rotor. The tap reduces the inductance of the turns in the coil to supply “high speed peak current” to the coils at high speed. Additional taps may be included to extend the range of speed while maintaining constant power output.
Although the system of the '668 publication maintains a constant power output over a range of machine speeds, it may be unreliable. For example, the system of the '668 publication relies on an electrical “tapping” of the coils of the electric motor, which requires additional conductors in close proximity with energized coils. These additional conductors may generate additional heat in the coils, create multiple high voltage potential points with the stator conductor, and/or otherwise unpredictably alter the operation of the coils that could increase the likelihood of premature coil conductor damage and/or inefficient operation of the motor. Furthermore, in multi-phase machines, tap locations must be located in the same relative position on each phase coil to ensure that the inductance reduction (and associated current flow) will be identical between the phases. Designing and locating identical and repeatable tap locations for each coil within the motor may substantially limit motor design flexibility.
In addition, because the electric motor of the '668 publication requires physical taps, manufacturing costs may be expensive. For example, tapping of the phase coils during manufacture of the machine may require additional time, personnel, and/or material resources to precisely position the tap locations, to properly wind the conductors with respect to the tap locations, and to ensure that all “tapped” locations are accessible by maintenance personnel. This cumbersome manufacturing process may substantially increase motor manufacturing and design costs.
Finally, the electric motor described in the '668 publication may be extremely difficult to repair and/or maintain. For example, should one or more of the tap conductors require replacement (or if additional taps need to be made), the phase coil must be unwound and/or replaced, which may require substantial technical knowledge. Furthermore, isolating a problematic tap could require substantial testing, potentially rendering the electric motor inoperable (and unproductive) for prolonged periods.
The presently disclosed electric motor is directed to overcoming one or more of the problems set forth above.